1. Field of the Invention
The invention relates to a feedback controller for NC controlled machine tools, particularly to a feedback controller that suppresses oscillating reaction of feedback control systems caused by periodic load fluctuation.
2. Description of the Related Art
In the field of machining, various kinds of machine tools have been used and recently NC controllers that control machine tools according to the numerical controlling information are widely used. The NC controllers often use feedback control (servo control). In the feedback control the movement of a work is adjusted by the error obtained by comparison of a predetermined desired value and the current value of the movement. Therefore, with a feedback controller, the position and the velocity of the work are adjustable by monitoring its position and velocity.
FIG. 5 shows the conventional configuration of a feedback controller of machine tools. In the figure, a driving motor 10 supplies a predetermined driving force for a work 12 to be machined or its bed on which the work 12 is fixedly mounted. The position detector 14 connected to the driving motor 10 detects the position of the work 12 in the straight or rotating movement (for example, the distance from a starting point on a base line, or the angle of rotation). In the figure, a velocity detector 16 detects the velocity of the work 12. In this example the velocity detector 16 gets a position detection signal 102 output from a position detector 14, and converts it into a velocity detection signal by differentiation.
As shown in the figure, a position director 18 generates a positioning demand that reflects the desired position of the work 12 which is based on a predetermined machining program. A position demand signal 101 output from the position director 18 is input into the first difference detector 20 that outputs the difference of the established desired position and the detected current position of the work 12. The first difference detector 20 outputs the difference of the established desired position and the detected current position of the controlled work 12 by comparing both values. A position difference signal 103 supplied from the first difference detector 20 is input to the position adjustor 22 which converts the position difference signal 103 into a velocity signal 104 in order to move the work 12 to the desired position with appropriate velocity, using the amount of the position difference as the parameter.
The velocity signal 104 output from the position adjustor 22, along with the velocity detection signal 105, are input into the second difference detector 24 which calculates the difference of the signals and outputs a velocity difference signal 106. The velocity difference signal 106 is subsequently input into a velocity adjustor 26 which converts the velocity difference signal 106 into a torque signal 107 in order to adjust the velocity of the work 12. The torque signal 107 is input into a motor driver 28, amplified, and passed into the driving motor 10 that moves the work 12.
Therefore, with these feedback position control and feedback velocity control, the controlled work 12 can be moved to a desired position with a desired velocity.
However, the conventional feedback controller is sometimes unable to reduce recurrent load fluctuation properly, because the system also reacts for recurrent fluctuation in high reliability. For example, in case of end mill working using complex engine lathe equipped with a conventional feedback controller, the interval of the load fluctuation synchronizes with the response period of the feedback loop, and the movement of the work react synchronously, which can break the tool or decrease the working accuracy. This is because the work (which is fixed or rotating) receives intermittent load by the working tool. The conventional feedback controller can also cause a position error because the torque control by the driving motor 10 could not catch up with the external force from the tool.
FIG. 6 shows the position error of the work that can result from the unbalance between the load torque T.sub.L and the motor torque T.sub.M. In this figure, (A) shows the load torque (T.sub.L), and (B) shows the motor torque (T.sub.M) that opposes against the load torque (T.sub.L) in the opposite direction, further (C) shows the distance (position error) .DELTA.X from the desired position of the work. As shown in this figure, because of the delay of the motor torque against the intermittent force of the load torque, the position error (.DELTA.x) which decreases the working accuracy of the work is generated.
In order to suppress the oscillating reaction in the feedback control system caused by the recurrent external force, a conventional controller utilizes damping mechanism at movable parts of the tool. In other words, the reaction is suppressed by adding artificial load to the movable parts of machine tools using the viscous resistance of oil or sliding resistance of a disc brake.
FIG. 7 shows a complex engine lathe 30 with a disc brake that suppresses reaction of the feedback control system. A main spindle 34 pierces through a bed 32 of the complex engine lathe 30. A driving motor 36 is placed near the bed 32. Driving force generated by the driving motor 36 is transmitted to the main spindle 34 via a belt 38. The bed 32 is equipped with a chuck 40 that receives the force from the main spindle 34 and is positioned so that rotation in every direction is possible. The chuck 40 holds a work 42. Above the bed 32, there are a turret 44 that is movable in the direction A and B of the figure, and a rotating tool 46 attached to the turret 44. The complex engine lathe 30 has a disc brake system 47 comprising a disc with the main spindle 34 piercing through it, and a brake pad 50 surrounding the disc.
Using the complex engine lathe 30 with the above components, the work 42 receives external force intermittently and reaction of the feedback control system becomes periodic, for example, in the following cases: end mill working while fixing the position of the work 42; and cutting of the surface of a non-spherical work 42 by rotation.
For reducing these kind of reaction movements of the conventional feedback control system, the following method can be applied. A load caused by viscous resistance will be generated by holding the disc 48 with a brake pad 50. By the viscous resistance, the response characteristics of the feedback loop will change and the reaction in the system will be avoided. This method, however, makes the size of the apparatus bigger, causes mechanical loss of energy, and increase the cost.